Sub-Slab Depressurization: How Radon Mitigation Actually Works

Last updated: 2026-04-26

Sub-slab depressurization (SSD) is the dominant radon-reduction technology in Canada because it works. Properly installed, an SSD system reduces indoor radon levels by 50–99%, with most C-NRPP installers reporting 80–95% on a first-pass install. The mechanism is simpler than the acronym suggests: vent the soil gas before it can enter the house.

If your test came back above Health Canada's 200 Bq/m³ action level and you're trying to figure out what mitigation actually involves, this is the explainer.

The physics, in one paragraph

Radon enters a home because the home is at slightly lower pressure than the soil beneath it. Warm indoor air rises, escapes through the upper envelope, and creates a mild negative pressure at the foundation — the stack effect. Soil gas, including radon, follows that pressure gradient through floor cracks, the sump gap, plumbing penetrations, the slab-to-wall joint, even pores in the concrete. SSD reverses the gradient: a small fan pulls air from a sealed point beneath the slab and exhausts it above the roof, so the underside of the slab sits at lower pressure than the basement above. Soil gas flows out instead of in.

What's in a typical SSD system

Five pieces, and only the fan moves.

• Suction point. A 4–5 inch hole drilled through the basement slab into the gravel layer beneath. If the slab was poured directly on dirt — common in older Nelson homes — the installer excavates a small pit under the hole to give the fan something to draw from. One point covers most homes; large or partitioned slabs sometimes need two.
• PVC vent pipe. Typically 4-inch schedule 40 PVC, running from the suction point up through a closet or mechanical chase and out above the roofline, terminating away from windows and air intakes. Health Canada's Radon Reduction Guide sets the clearance specs.
• Inline radon fan. Mounted in the attic or on the exterior wall — never inside the conditioned envelope, because a leak in the housing would push radon-rich exhaust back into the home. Models like the RadonAway RP-Series draw 50–100 watts and run continuously.
• Manometer. A small U-tube water gauge on the vent pipe. It shows a pressure differential when the fan is working. If the column reads zero, the fan has failed.
• Sealing. Slab cracks, the sump cover gap, plumbing penetrations, and the slab-to-wall joint get sealed with polyurethane caulk or hydraulic cement. An unsealed sump or floor drain is the leak that defeats the system.

Slab, basement, crawlspace — same idea, different mechanics

The standard layout above assumes a poured concrete basement slab. Crawlspaces use sub-membrane depressurization (SMD) instead — a heavy-gauge polyethylene membrane (12–20 mil) laid across the dirt floor, sealed to the foundation walls, with the vent pipe and fan drawing from beneath it. Same physics, more labour because every seam has to be airtight.

Slab-on-grade homes — common in newer Kootenay builds with no basement — use the same SSD method as a basement install, with the vent pipe rising through a closet or chase to the roof. Bedrock-source radon doesn't care whether you have a basement; slab-on-grade homes test high too.

What installation day looks like

Most Kootenay SSD installs run one to two days. Drilling the suction point creates concrete dust — finished basements get a temporary plastic enclosure, bare basements get a tarp. Drilling itself takes 20–40 minutes. The installer then excavates the pit, sets the pipe stub, and seals it.

Running the pipe is the visible part. In unfinished basements and attics it's exposed; through finished spaces it routes via closets or chases, with drywall cuts patched but unpainted (paint is on the homeowner). On the exterior, the fan goes on a wall bracket or in the attic, and the pipe terminates above the roofline. The fan ties into a dedicated 15-amp circuit installed by a licensed electrician. By end of day two the system is running and the manometer shows a draw.

What it costs in the Kootenays

A typical Kootenay SSD project runs $2,500–$4,500. Crawlspace SMD systems are usually higher — $3,500–$5,500 — because membrane work is labour-heavy. A new dedicated electrical circuit for the fan adds $200–$500 if one isn't already available. Multiple suction points, finished-basement restoration, awkward vent routing, and tall homes push projects toward the upper end.

There are no provincial or federal rebates for radon mitigation in BC. CleanBC ESP, HRR, FortisBC, BC Hydro, and HomeSave Central Kootenays all exclude it. The Canadian Lung Association occasionally runs a small means-tested grant; budget as if it doesn't exist.

The post-install test

A C-NRPP installer's quote should include a post-mitigation confirmation test — a long-term alpha-track test (3 to 12 months) placed where the original test sat. The result tells you whether the system actually reduced your levels. If the reading is still elevated, the installer adjusts: sometimes a second suction point, sometimes a higher-static-pressure fan, sometimes more sealing. Skip this step and you have an expensive PVC pipe with no proof it solved the problem.

Maintenance and operating cost

The fan is the only moving part, and the only thing that fails. Service life is 10–15 years; replacement runs $300–$500 plus a service call. The pipe lasts the life of the house. Annual maintenance is one visual check that the manometer still shows a pressure differential.

A 50–100 watt fan running continuously adds $50–$150 a year to a Nelson Hydro bill. Trivial compared to the lung-cancer risk reduction.

What about an HRV instead of SSD?

A heat-recovery ventilator dilutes indoor air, including radon, by exchanging it with outdoor air. For homes that test moderately above the action level — say 220–300 Bq/m³ — an HRV on a higher fan setting can sometimes pull levels below 200. For homes well above that, HRV alone is rarely enough.

The reason is mechanical. SSD removes radon at the source, before it enters the home. HRV dilutes it after entry. Pushing enough airflow to dilute a 600 Bq/m³ home below the action level is energetically expensive and noisy. SSD just stops the inflow.

For homes pursuing both an envelope retrofit and radon mitigation, an HRV plus SSD combination is sometimes the right answer — the HRV provides the ventilation a tighter envelope needs, and the SSD handles radon directly. See our radon pillar for the cross-pillar sequencing on insulation and heat-pump retrofits, and the radon test guide for the testing protocol that should come first either way.

FAQ

Will I see the pipe inside my house?

Some of it. In unfinished basements and attics the pipe is exposed; through finished spaces it routes through closets or chases with drywall patched but unpainted. On the exterior it runs up to the roofline with a rain cap. Most homeowners stop noticing after a week.

How long until radon levels actually drop?

The fan starts pulling immediately. Short-term electronic monitors (Airthings, Corentium) usually show the reduction within 24–48 hours. The post-install long-term test takes 3–12 months for a confirmed average.

Does an SSD system change anything else about my home?

Not in any way the homeowner notices day to day. The fan is quiet, it doesn't affect heating, cooling, or humidity, and the suction is below the slab — not above it.

What if my fan fails on a Sunday in January?

The manometer reads zero. Radon levels rise back toward baseline over days, not minutes, so a week of fan downtime isn't an emergency. Call the installer Monday — most carry replacement fans in stock.

Can I install an SSD system myself?

Technically possible, practically inadvisable. A non-C-NRPP install loses the diagnostic standards, the post-install confirmation protocol, and any insurance recourse. Health Canada and the Canadian National Radon Proficiency Program both recommend certified installers for residential mitigation.

Kootenay Energy doesn't install radon systems — we match you with vetted local C-NRPP installers and stay involved if the project sits inside a broader retrofit. If you're also planning insulation or a heat pump, our calculator maps the rebate stack on the energy side and we coordinate the radon piece alongside it.